The present invention relates to a return spring which is employed, for example, in the multiple disc clutch of an automobile's automatic power transmission.
A multiple disc clutch employed in an automatic power transmission or the like comprises a housing, a clutch piston, a number of input friction discs, and a number of output friction discs. Inside the housing, the input friction discs and the output friction discs are alternately arranged in such a manner that they can come into contact with each other and be separated from each other. When the clutch piston is hydraulically driven, the input friction discs and the output friction discs engage with each other, with the result that the clutch is let in (clutch meet). The clutch device incorporates a return mechanism. When the hydraulic pressure exerted on the clutch piston becomes lower than the predetermined value, the return spring of the return mechanism moves the piston back to the original position. When the piston returns to the original position, the input friction discs and the output friction discs are disengaged from each other, with the result that the clutch is released.
A variety of return springs, which are to be incorporated in multiple disc clutches, are designed in accordance with the types of automobiles. The retainer assembly 3 shown in FIG. 8A is a return spring of one type. In the retainer assembly 3, a plurality of coil springs 1 are arranged in the circumferential direction of retainers 2. An example of such a retainer assembly is disclosed in Jpn. UM Application KOKOKU Publication No. 1-26914. The spiral wave spring 4 shown in FIG. 8B is also known as being usable as a return spring. The spiral wave spring 4 is made of a flat spring material, and this material is worked in such a manner as to form a wave pattern. An example of such a spring wave spring is disclosed in Jpn. UM Application KOKOKU Publication No. 6-10226. The compression coil spring 5 shown in FIG. 8C is still another example of a return spring.
In these types of return springs, the distribution of load must be as uniform as possible at the contact portion between the bearing surface of a spring and the end face of a clutch piston. In a clutch piston used in this manner, the dimension in the axial direction is much smaller than the piston diameter. This being so, the distribution of load must be as uniform as possible in the bearing surface of the return mechanism. If the distribution of load is not uniform, it is likely that the clutch piston will tilt with reference to its corresponding structural member (i.e., the inner face of the cylinder). If the clutch piston tilts, the following problems (a) to (d) may occurs
(a) The piston comes into tight engagement with its corresponding structural member and does not move. PA1 (b) The piston does not smoothly move, resulting in the generation of noise (so-called squeaking noise). PA1 (c) A force is applied concentratedly to the sliding contact portion between the piston and the corresponding structural member, resulting in non-uniform abrasion. PA1 (d) Since the piston tilts even when the residual pressure is low, it may not completely return to the original position, resulting in the occurrence of a clutch drag. PA1 (1) The piston is prevented from coming into tight engagement with its corresponding structural member. PA1 (2) When the piston is operated, it does not generate noise, such as squeaking noise. PA1 (3) A force is not concentratedly applied to the sliding contact portion between the piston and the corresponding structural member, and local abrasion is suppressed thereby. PA1 (4) Since the clutch operates with high responsiveness even to a low residual force (hydraulic force), a so-called clutch drag can be prevented. PA1 (5) Since the single coil spring is made of a wire with a simple cross sectional shape and can be used without being combined with another member, it is very simple in structure in comparison with a conventional retainer assembly or a conventional spiral wave spring. Since it can be easily assembled with reference to a clutch, a clutch incorporating the subject return spring can be realized at low cost.
Of the three types of return springs mentioned above (namely, the retainer assembly 3, spiral wave spring 4, single compression coil spring 5 shown in FIGS. 8A, 8B and 8C), the compression coil spring 5 is advantageous in that it can be incorporated in a clutch at low cost since it is simpler in structure than the others and can be manufactured and assembled at low cost. However, the spring storage space inside the clutch is very narrow. In particular, the spring storage space is very restricted in dimension in the axial direction of the compression coil spring. Although the compression coil spring has to be made of a wire whose diameter is large enough to satisfy the required spring constant, such a coil spring cannot be stored inside the clutch unless the number of effective turns is small In fact, a compression coil spring whose effective turns are two or more cannot be used as a spring coil incorporated in a clutch
It should be noted that a compression coil spring with a small number of effective turns has problems in that the eccentricity (eccentric distance) between the center of the coil spring and the center of load is long and the distribution of load in the bearing surface is therefore likely to be non-uniform. In the case of a conventional compression coil spring having an open-ended end turns its end-turn portion is normally designed to have 0.7 turns or more, so as to ensure a stable seated condition when no load is applied (i.e., when the compression coil spring has a free length) However, if the effective turns are two or less and the end-turn portion has 0.7 turns or more, the eccentricity between the center of the coil spring and the center of load is inevitably long, and the use of such a compression coil spring gives rise to the problems (a) to (d) mentioned above. It is therefore not undesirable to use such a coil spring as a return spring to be incorporated in a clutch.